This invention relates to a process and apparatus for recording images on an electrographic medium and in particular to an improved process and apparatus for recording images on an electrographic medium that has a field permeable conductive layer. A field established behind the rear surface of the medium opposes a field established at the front, or charge retaining, surface of the medium so as to counteract any tendency of the charge on the charge retaining surface of the medium to be distributed in an undesired pattern as a result of defects in the medium.
In one type of electrophotographic recording process, the recording medium is a film having a polyester base, a transparent conductive layer that overlies the polyester base and a photoconductive layer that overlies the transparent conductive layer. To record on the film, a uniform electrostatic charge is first applied, by means of a charging corona element, to the surface of the photo conductive layer of the film. The uniform charge is then selectively dissipated by exposing the surface to a light image of the pattern to be recorded. The resulting pattern of charges is an electrostatic latent image on the surface of the photoconductive layer which can then be rendered visible, for example, in an attraction toning process, by applying thereto electrostatically charged developer particles which are held to the surface of the photoconductive layer by means of the electrostatic force developed between the developer particles and the charge on the surface on the photoconductive layer. A permanent visible image can be obtained, for example, by using developer particles which can be heat fused to the photoconductive layer, and then subjecting the visible image to a heat fusing step.
It is possible for the manufactured film to have defects. Defects can take several forms such as an irregularity in the thickness of the photoconductive layer of the film, or a piece of foreign matter within the photoconductive layer, or a small pinhole in the photoconductive layer of the film. These defects prevent uniform charging of the surface of the photoconductive layer during the charging step of the recording process. Generally, the area of the photoconductive layer at the defect leaks electrically, that is, the area is unable to maintain a desired level of charge. As a result, the area of the defect is at a lower potential than the area surrounding the defect. Because of this potential gradient, a non-uniform field is established in the vicinity of the defect that tends to divert charge, that would otherwise be deposited on the surface of the photoconductive layer to the defect. In some cases, the dissipation of the charge at the defect results in a sufficiently large current level which can vaporize either the delicate conductive layer or the photoconductive layer at that point. In a similar manner, these defects can cause toner particles to be distributed in an undesired pattern in the area of and surrounding the defect. Since the undesired distribution of charge or toner particles occurs in an area surrounding the defect, undesired images are recorded on the film over an area that greatly exceeds the area of the defect. If the undesired image is formed in an area of the film on which a desired image is to be recorded, the recording may be rendered useless.
Although the ideal solution to the defect problem is to provide film that does not have any defects, it would be very desirable for the recording system to be able to minimize this undesired distribution of charge caused by defects in the film. For example, if the undesired distribution of charge or toner particles in the vicinity of a defect could be reduced, the likelihood that an undesired recorded image resulting therefrom would interfere with the recording of a desired image would also be reduced. Furthermore, reducing the initial concentration of charge at the defect may prevent the ultimate vaporization of the film in that area and the film may satisfactorily record an image thereon. In either case, there will be some savings of film material as well as a reduction of the time and effort involved in rerecording images.
It has been observed that if a field is established from the insulative base side of the film that opposes the field established by the charging corona or the field tending to deposit toner particles on the film, the resulting developed image is improved in the vicinity of the defect. It has also been observed that the resulting developed image is considerably poorer in the vicinity of the defect if a field of the opposite polarity is established from the base side of the film. It is believed that the improved recording results because the field established on the base side of the film is able to penetrate the conductive layer of the film. It is also believed that the penetrating field has a greater effect in the vicinity of a defect than in non-defective areas and that the penetrating field, in effect, modulates the field applied from the charged side of the film so as to result in a more uniform field on the charged side of the film, than would exist in the absence of the penetrating field. Since the field on the charged side of the film is more uniform, the tendency to deposit charge or toner particles in an undesired pattern in the vicinity of a defect is reduced.
It is, therefore, the primary object of this invention to provide an improved electrographic recording apparatus and process that will reduce the undesired distribution of charge on a charge retaining surface of a recording medium in the vicinity of a defect in the medium.
It is another object of this invention to provide an improved apparatus and process for recording on an electrographic medium that will reduce the undesired distribution of charge on a charge retaining surface of the medium in the vicinity of a weakened area or hole in a conductive layer of the medium.
And yet another object of this invention is to provide an improved apparatus and process for recording on an electrographic medium that will reduce the undesired distribution of toner particles on the surface of the medium in the vicinity of a defect in the medium.